Rapid cytometric assays for lymphocyte activation have many potential clinical applications, including evaluation of cellular immune dysfunction in HIV infection and cancer, assessment of the activity of autoimmune disease processes, tissue typing, and monitoring of transplant recipients for early signs of graft rejection. With a view toward the development of reproducible, standardized methods which can be implemented on the current generation of clinical flow cytometers, as well as on newer cytometric apparatus now coming into use, multiparameter flow cytometry will be used to analyze and correlate changes in cytoplasmic pH, fluorescein fluorescence polarization, and activation antigen expression of human peripheral T-lymphocytes during the first 24 hours following activation by the polyclonal mitogen phytohemagglutinin or by alloantigen in the mixed lymphocyte reaction. It is likely that the proposed studies will yield usable assays based on pH measurement and activation antigen detection.